The invention concerns a vaporizer crucible for vacuum vapour-deposition apparatus installations and comprising a metallic body which has a recess for accommodating vaporizing material and connected in a thermally-conductive manner to a cooling duct which surrounds the recess at least partially.
Vaporizer crucibles of the above kind are disclosed in, for example, DE-PS No. 12 62 101. Therein, the cooling duct is formed between an inner wall and an outer wall, but the flow of cooling medium is left more or less to chance, i.e., the stream of cooling medium follows the path of least resistance.
Fluid-cooled vaporizer crucibles of this kind are generally used for apparatus in which the vaporization material is heated directly, by electron bombardment for example, since direct heating through the wall of the crucible is precluded because of the cooling of the fluid. Electron bombardment is generally carried out from above, so that because of the danger of electrons impinging upon it the upper edge of the crucible is, in fact, the critical part of the crucible that has to be cooled in a particularly intensive manner. With the known double-walled crucible, very considerable rates of flow and therefore high consumption of coolant e.g. water are required in order to ensure adequate and reliable cooling.
Vaporizer crucibles for electron beam heating are also known which consist of a substantially solid block of metal (copper) having high thermal conductivity, in which block is provided a cooling duct which surrounds the recess and is of roughly constant cross-section. A cooling duct of this kind is formed by, for example, a plurality of drilled holes of like diameter which are disposed at right angles to each other; the drilled holes communicate with each other and, apart from two connection openings, are closed off from the exterior by means of plugs. The flow in cooling ducts of this kind must follow the path of the duct system, i.e. short-circuiting of the flow is precluded. However, a further problem remains, namely the formation of vapour bubbles that occur during the heavy loading of the crucible by the electron bombardment.
In the cooling of vessels comprising cylindrical or conical recesses for accommodating vaporization material, at least one cooling duct is expediently provided concentrically to the recess. When cooling medium flows through this duct, the centrifugal force which develops results in force being applied to the coolant in the direction towards the outer periphery of the cooling duct. If the cooling medium contains constituents, such as the above-described vapour bubbles, that have a lower specific weight than that of the cooling medium, these constituents are displaced toward the inner wall of the annular cooling duct (centrifugal effect). In the case of cooling circuits wherein the heat transferred per unit surface is great and in which what is called "bubble vaporization" occurs, this effect can drastically reduce the heat transfer at the inside, i.e. calculated at a zone where the greatest thermal loading of the material of the crucible takes place.
However, even with cooling ducts which are formed by a system of rectilinear drilled holes, any vapour bubbles that may be formed rise, under the effect of the force of gravity, to the line of the cylinder constituted by each drilled hole, and in this zone they drastically reduce heat transfer. Heavy thermal loading occurs also in this zone, particularly when electrons impinge upon the upper edge of the crucible.
In practice, depending upon the path of the cooling duct a resultant between centrifugal force and force of gravity is then established and, particularly with crucibles having a conical recess, this leads to disadvantages, since the danger of tapered crucible walls being acted upon by electrons increases as the quantity of vaporization material diminishes. Crucibles having conical recesses are desirable, however, since under the effects of electron bombardment, the vapour stream spreads upwardly, roughly in the form of a club, and as far as possible it should not be allowed to come into contact with the edge of the crucible, so as to avoid shading effects.